The invention relates to a heat-hardenable binder mixture based on a hydroxyl-containing organic resin.
Many and varied chemical reactions have been proposed, and in fact used, for hardening paints by cross-linking the binders in paint films. The chemical bonds formed in the course of the crosslinking reaction frequently do not meet all the demands made on paint films.
In 2-component systems, it is frequently necessary to block one of the reactive groups to suppress premature reaction. The compounds then released in the course of baking pollute the environment, in particular in the case of amines or phenols.
Electrocoating has become widely established in recent years for priming electrically conductive substrates. Anodic electropriming has so far been to the fore. The resin binders used for this purpose belong to the class of carboxyl-containing resins, for example to the classes of maleinate oils, maleinated epoxy resins, alkyd resins, acrylic resins and in particular maleinated polybutadienes. These resins were made water-soluble by salt formation, chiefly with amines, and deposited in the electrocoating bath by the current at the anode. The anodic electropriming process, however, has serious disadvantages. For instance, oxygen evolves in the course of the electrodeposition at the anode and can have a drastic, adverse effect on the resins depositing at the anode. Furthermore, metal ions go into solution at the anode and end up in the baked film as imperfections. The metal ions can bring about discoloration and spots. They are the cause of qualitative disadvantages, in particular through salt formation and hence through reduction in water resistance and corrosion protection.
Cathodic electropriming, which has been developed in recent years to the stage of commercial feasibility, is increasingly replacing the anodic method, since the deficiencies described above are largely avoided. For instance, the cathode, where the paint film is now deposited, evolves, in the course of the deposition process, hydrogen, which does not affect the resin binder. Since cathodic deposition can take place within the approximately neutral pH range, hardly any metal ions go into solution. The binders suitable for cathodic deposition predominantly contain amino groups which are neutralized with acids to make the resins water-soluble.
European Pat. No. 12,463 B 1 discloses binders which crosslink with hydroxyl-containing resinlike compounds via the transesterification of crosslinking agents containing .beta.-hydroxyalkyl ester groups. This transesterification, however, requires large amounts of heavy metal salts as hardening catalysts.